In the Actiniæ, you will find the loose disorderly filaments changed into feelers, with wonderfully acute sensation, regularly disposed in a star. When a substance touches the feelers they close in with considerable power, but seem to act more like a mechanical trap,--as the leaves of certain plants catch flies and other insects,--than in direct dependance on the will of the animal. However, the Actinia has the sense of feeling, and has a perception of light and of odours, though without eyes or nose. Having greater muscular strength and more compactures of film, he does not need the destructive fluid with which the Acalepha is furnished. Thus, one sort of power is made to compensate for the loss of another.
The Sepia also catches his food by means of fleshy organs placed round his mouth; but in him you find them possessed of amazing power, moving in strict subjection to his will; never loosely floating about as in the others. Then he tastes, sees, and hears, by means of a tongue, eyes, and ears, distinctly formed.
A similar gradation is observed in the development of the various parts composing the hand and arm in animals with perfect skeletons, in which the bones act as levers. There is a very interesting book[C] written on this subject, which you would do well to read; and you will see in it that all sorts of birds, quadrupeds, and even fishes, have, in their fore feet, wings, and fore fins, elements resembling those of the human hand and arm. Often, in comparing only two animals, you might fail to trace the slightest resemblance; but when one or two or more of other kinds are placed between them, you find a sort of ladder which, in an evident manner, unites the lowest with the highest tribes.
[Footnote C: Sir Charles Bell's Treatise on the Hand.]
At distant periods of the earth's history, you may see the same sort of organs, and the same dependance of the creatures one on another. The fierce Icthyosaurus, and the sly, long-necked Plesiosaurus, had eyes, ears, tongues, and other parts, the same as our contemporaries; and they ate and digested their food, and moved from place to place, and preyed on each other in no other manner.
If our acquaintance with nature were much greater than it is, we should doubtless be able to bring proofs that there is no sort of stones, of vegetables, or of animals, nor any process or movement of the elements, in which we have not an interest. There is no fact that is not in some way or other connected with the whole, so as to influence its well-being.